The exit day – 11pm GMT on March 29, 2019 – will be included on the EU withdrawal bill going through parliament.
Brexit Secretary David Davis said the change will make the UK's departure date "crystal clear".
But the author of the Article 50 process, Lord Kerr, has insisted Brexit could still be reversed, warning the British people are in danger of being misled.
The cross-bench peer, who played a key role in drafting the legal mechanism for a country to leave the EU, will say in London today: “We can change our minds at any stage of the process.”
At an event hosted by the pro-EU Open Britain, he is set to say "we are not required to withdraw just because Mrs May sent her letter".
It comes after the Prime Minister issued a blunt message to pro-EU MPs, saying any attempt to derail Brexit will not be tolerated.
Writing in the Daily Telegraph, Mrs May said: "We will not tolerate attempts from any quarter to use the process of amendments to this Bill as a mechanism to try to block the democratic wishes of the British people by attempting to slow down or stop our departure from the European Union."
Mr Davis is today set to meet the European Union's chief negotiator Michel Barnier in Brussels, where officials have been engaged in the latest round of Brexit talks.
Mr Davis indicated that he was talking a "pragmatic" approach to MPs' concerns by writing Brexit day into the Bill.
He said: "Our amendment makes it crystal clear that the UK is leaving the EU at 11pm on March 29 2019.
"We've listened to members of the public and Parliament and have made this change to remove any confusion or concern about what 'exit day' means.
"This important step demonstrates our pragmatic approach to this vital piece of legislation.
"Where MPs can improve the Bill, whatever their party, we will work with them.
"We look forward to further debate in the House of Commons when committee stage begins next week."
Ahead of the meeting in Brussels, Mr Barnier called on the UK to make clear whether it will stick with the "European model" on issues like food and environmental standards and financial regulation when it leaves the bloc.
With a crunch leaders' summit in December, Mr Barnier also said the moment was approaching for a "real clarification" of Britain's position on issues like citizens' rights, the Irish border and the UK's financial settlement.
If the 27 remaining EU members agree next month that sufficient progress has been made on these issues, they will give a green light for negotiations to move on to the questions of trade and transition to a new post-Brexit negotiation.
The Prime Minister will attempt to win support from European businesses for her goal of moving the negotiations on to trade talks.
She will meet leading business organisations on Monday to set out her vision of a "bold and deep economic partnership" between the UK and EU after Brexit.
The Brexit Secretary and Business Secretary Greg Clark will also attend the Downing Street event, which the CBI has helped to organise.
Groups represented include the BDI and BDA from Germany, Medef from France and the EU-wide BusinessEurope.
Ministers hope that business groups can lobby their national governments to support a trade deal which will be in the interests of both sides in the negotiations.
But in a sign of business unease about the impact of Brexit, motor manufacturers Honda, Ford and Vauxhall set out their concerns in evidence to a Commons select committee.
In a written submission to the Business, Energy and the Industrial Strategy committee, Honda, which employs 4,000 people at its Swindon plant, said it had concerns about disruption to supply chains, regulatory divergence, restrictions on free movement of labour and the impact of tariffs.
"With 35 per cent of exports from our Swindon factory exported to the EU, and 40 per cent of the components used to build cars at Swindon imported from EU27 suppliers, frictionless, tariff-free access to the single market and customs union is vital to our competitiveness," the Japanese motor giant said.
Honda also warned that non-tariff barriers could potentially have a greater impact than import and export tariffs.
Ford said any transitional deal should be a "standstill agreement" preserving current rules because "we do not believe that the new customs, border and documentation arrangements necessitated by the UK leaving the EU and customs union could be put in place by April 2019".